Experimental results are used to determine the dependence of bubble density, at grain boundaries as well as within the grains, on irradiation temperature and helium gen eration rate. The measured flux of helium to grain boundaries is then related to helium generation rate. Finally, the dependence of the grain boundary bubble density on helium flux is obtained. These dependencies point to a possibility that under irradiation the diffusion kinetics of helium during the bubble nucleation phase may be material independent.
Analytical calculations are made to determine these dependencies. In these calculations, the nucleation at the boundary and within the grains is considered to be coupled. The comparison of the calculated and experimental results shows that the dependence of the average flux of helium on the helium generation rate can be predicted reasonably accurately within the framework of the diatomic type nucleation. The nucleation behavior at grain boundaries has been calculated using diatomic and multi-atomic type mechanisms. Dependencies calculated in terms of the multi-atomic type nucleation are found to be consistent with the observed nucleation behavior at grain boundaries.